Chronic one-kidney hypertension was produced in rats by constriction of one renal artery and removal of the opposite kidney. The elevated blood pressure in 20 of 25 of these rats was significantly lowered by the subcutaneous injection of extracts of rabbit or hog kidney. In 16 of the 25 rats, blood pressure fell to normal levels during the immunization period. Although some of these rats developed antirenin titers against rabbit or hog renin, none developed antirenin against rat renin. There was no correlation between the lowering of blood pressure and the development of antirenin titers. Antirenin produced in normal rats against purified rabbit or hog showed very little, if any, cross-reaction with rat renin. Thus, it is unlikely that the low concentrations of anti-rabbit or anti-hog renin which developed in the hypertensive rats had any effect on the rats' own circulating renin. The results indicate that a factor other than renin is essential for maintaining the elevated blood pressure in the one-kidney hypertensive rat. The lowering of blood pressure appears to be a result of the neutralization of this factor by antibody against an immunologically-similar antigen present in rabbit and hog kidney.